Chemically Derivatized Semiconductor Photoelectrodes - Advances in

15 Chemically Derivatized Semiconductor Photoelectrodes

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A Technique for the Stabilization of n-Type Semiconductors 1

MARK S. WRIGHTON , ANDREW B. BOCARSLY, JEFFREY M. BOLTS, MARK G. BRADLEY, ALAN B. FISCHER, NATHAN S. LEWIS, MICHAEL C. PALAZZOTTO, and ERICK G. WALTON Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139

Pretreated Au, Pt, n-type Si, and n-type Ge can be deriva tized with trichlorosilylferrocene, (1,1'-ferrocenediyl)dichlorosilane, and 1,1'-bis(triethoxysilyl)ferrocene to yield elec troactive, surface-attached, oligomeric ferrocene material. Derivatized, n-type semiconductors exhibit photoeffects expected for such an electrode material; irradiated deriva tized n-type Si can be used to effect the oxidation of solution reductants by mediated electron transfer, unique proof for which comes from the semiconductor electrode that responds to two stimuli, light and potential. The sustained, mediated oxidation of Fe(CN) in aqueous solution in an uphill sense by irradiation of derivatized n-type Si is pos sible whereas a naked n-type Si undergoes decomposition to SiO at a rate too fast to allow sustained energy conver sion. This establishes the principle of manipulating inter facial charge-transfer kinetics for practical applications. 4-

6

x

S

emiconductor-based

p h o t o e l e c t r o c h e m i c a l cells h a v e p r o v e d t o g i v e

t h e h i g h e s t efficiency o p t i c a l t o c h e m i c a l ( 1 , 2 , 3 ) a n d e l e c t r i c a l ( 4 )

e n e r g y c o n v e r s i o n o f a n y w e t c h e m i c a l system. T h e h i g h e s t solar e n e r g y c o n v e r s i o n efficiency c l a i m e d ( 4 ) thus f a r is 1 2 % f o r a n n - t y p e G a A s 1

Author to whom inquiries are to be addressed. 0-8412-0474-8/80/33-184-269$06.75/0 © 1980 American Chemical Society Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

270

INTERFACIAL

PHOTOPROCESSES

b a s e d c e l l e m p l o y i n g a n a q u e o u s e l e c t r o l y t e s o l u t i o n of S e " . S u s t a i n e d n

2

c o n v e r s i o n of l i g h t to e l e c t r i c i t y i n a l i q u i d j u n c t i o n d e v i c e e m p l o y i n g a n o n - o x i d e n - t y p e s e m i c o n d u c t o r has d e p e n d e d o n t h e d i s c o v e r y of r e d u c t a n t s t h a t are c a p a b l e of c a p t u r i n g p h o t o g e n e r a t e d rate t h a t p r e c l u d e s p h o t o a n o d i c Photoanodic

decomposition

decomposition

of t h e

is e n e r g e t i c a l l y p o s s i b l e

(5-12)

holes at a

semiconductor. for

(13,14)

any

n - t y p e s e m i c o n d u c t o r i m m e r s e d i n a l i q u i d e l e c t r o l y t e exposed t o b a n d g a p , or greater, e n e r g y l i g h t . F o r a l l non-oxides s t u d i e d thus f a r , H 0 is 2

n o t o x i d i z e d i n t e r f a c i a l l y at a rate t h a t c o m p e t e s w i t h the o m n i p r e s e n t photoanodic

decomposition.


T h e t e c h n i q u e of n - t y p e s e m i c o n d u c t o r s t a b i l i z a t i o n b y a d d i n g r e d u c t a n t s to the s o l u t i o n does not a l l o w t h e e l e c t r o d e

to b e u s e d

to

d i r e c t l y d r i v e a n y other o x i d a t i o n r e a c t i o n other t h a n t h e o x i d a t i o n of t h e a d d i t i v e . B y w a y of contrast, the k i n e t i c a l l y i n e r t n - t y p e s e m i c o n d u c t i n g oxides s u c h as T i 0

c a n b e u s e d to effect a n u m b e r of

2

i n c l u d i n g o x i d a t i o n of H 0 (15-20) 2

a n d h a l i d e s (21,22).

oxidations

B u t t h e oxides

suffer f r o m e i t h e r h a v i n g s u c h a l a r g e b a n d gap t h a t o n l y short w a v e l e n g t h l i g h t is effective

or h a v i n g a set of e n e r g y

m a t c h e d to t h e r e d o x reactions of interest (23,24). cells d o n o t r e q u i r e r i g o r o u s p r o t e c t i o n f r o m 0

levels i m p r o p e r l y Inert

oxide-based

as d o cells e m p l o y i n g

2

h i g h l y r e d u c e d s t a b i l i z i n g reagents s u c h as S ", S e " , a n d T e * 2

2

(4,5-11).

2

N o n a q u e o u s e l e c t r o l y t e solutions m a y offer some advantages w i t h respect to b o t h energetics a n d k i n e t i c s at the i n t e r f a c e (12,13,25-28), solution conductivity compared

w i t h aqueous

potential difficulty. A l s o , i n nonaqueous from both 0

2

but lower

e l e c t r o l y t e systems is a

e l e c t r o l y t e systems

protection

a n d H 0 m a y b e r e q u i r e d f o r l o n g t e r m constant o p e r a t i o n . 2

I n this c h a p t e r w e o u t l i n e o u r results c o n c e r n i n g a n e w

technique

a i m e d at u l t i m a t e l y y i e l d i n g stable s e m i c o n d u c t o r - l i q u i d interfaces f o r optical energy transduction.

B a s i c a l l y , o u r a p p r o a c h is to

covalently

a t t a c h a r e d u c i n g reagent ( A ) to t h e surface of t h e s e m i c o n d u c t o r that the photogenerated hole r a p i d l y yields A , w h i c h i n turn +

s o m e s o l u t i o n r e d u c t a n t (B)

f o r m i n g B\ thus r e g e n e r a t i n g t h e

such

oxidizes surface

r e d u c t a n t . T h e c r u c i a l difference b e t w e e n a " n a k e d " a n d a " d e r i v a t i z e d " electrode is t h a t t h e net o x i d a t i o n , B -> B , is effected b y a h o l e l o c a l i z e d +

i n s e m i c o n d u c t o r e l e c t r o n i c levels i n t h e f o r m e r a n d b y a discrete m o l e c u l a r o x i d a n t i n t h e latter. W h i l e the p h o t o g e n e r a t e d is a sufficiently p o w e r f u l o x i d a n t t h a t t h e B - » B

+

h o l e i n e i t h e r case

r e a c t i o n is t h e r m o d y

n a m i c a l l y p o s s i b l e , t h e k i n e t i c s for net o x i d a t i o n of B to B w i l l b e d i f +

ferent i n t h e t w o cases.

A n important advantage

of the d e r i v a t i z e d

electrodes is t h a t t h e s t r u c t u r e of A c a n b e v e r y w e l l k n o w n , since A m a y b e a s m a l l m o l e c u l e . M a n i p u l a t i n g t h e n a t u r e of A , a n d h e n c e t h e n a t u r e of t h e surface e x p o s e d to t h e s o l u t i o n , m a y result i n m a j o r changes i n t h e k i n e t i c s of t h e net i n t e r f a c i a l c h a r g e - t r a n s f e r reactions.

Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

15.

WRIGHTON E T AL.

Derivatized

E l e c t r o a n a l y t i c a l (25-28)

Semiconductor

271

Photoelectrodes

a n d surface a n a l y s i s (29,30,81)

indicate

t h a t e l e c t r o n i c levels at t h e surface f a c i l i t a t e e l e c t r o n transfer to s o l u t i o n species.

S u c h surface states m a y c o n t r o l b o t h energetics a n d k i n e t i c s .

Derivatizing

electrode

surfaces

with

electroactive

molecules

can

v i e w e d as a d e s i g n e d i n t r o d u c t i o n of surface states t h a t c a n b e

be well

c h a r a c t e r i z e d f r o m t h e p o i n t of v i e w of s t r u c t u r e , k i n e t i c s , a n d energetics. D e r i v a t i z e d surfaces are i n t e r e s t i n g i n a d d i t i o n a l w a y s r e l a t e d to t h e s t a b i l i z a t i o n a n d efficient u t i l i z a t i o n of s e m i c o n d u c t o r - l i q u i d interfaces. F i r s t , d e r i v a t i z a t i o n of t h e surface w i t h m o l e c u l e s e n d o w s t h e surface w i t h m o l e c u l a r specific p r o p e r t i e s . A s a p r o t o t y p e , note t h a t d e r i v a t i z a Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch015

t i o n of a c o n v e n t i o n a l e l e c t r o d e surface w i t h a c h i r a l m o l e c u l e results i n a n a b i l i t y to p r o d u c e o p t i c a l l y a c t i v e e l e c t r o c h e m i c a l p r o d u c t s Second, the achievement

(4)

(32).

of a 1 2 % - e f f i c i e n t , n - t y p e G a A s - b a s e d

l i q u i d j u n c t i o n c e l l d e p e n d s o n a surface p r e t r e a t m e n t t h a t changes t h e i n t e r f a c e e l e c t r o n i c states so t h a t a h i g h e r o u t p u t v o l t a g e c a n b e o b t a i n e d . D e r i v a t i z a t i o n m a y y i e l d s i m i l a r effects, i n a d d i t i o n to a l l o w i n g d e s i g n e d m a n i p u l a t i o n of c h a r g e - t r a n s f e r k i n e t i c s . F i n a l l y , b y p h y s i c a l l y separat i n g t h e s e m i c o n d u c t o r surface f r o m t h e l i q u i d a n d / o r t a k i n g a d v a n t a g e of h y d r o p h o b i c - h y d r o p h i l i c b a r r i e r s to c h a r g e transfer a n d solvent acces s i b i l i t y , d e r i v a t i z a t i o n of surfaces w i t h p o l y m e r s ( e l e c t r o a c t i v e or n o t ) m a y a l l o w t h e d e s i g n of interfaces h a v i n g v e r y different p r o p e r t i e s . F o r e x a m p l e , m i c e l l e s h a v e d e m o n s t r a t e d (33)

efficient p h o t o i n d u c e d elec

t r o n transfer w i t h s l o w b a c k r e a c t i o n , b u t h a v e n o t b e e n p r a c t i c a l l y u s e f u l since t h e p h o t o s e p a r a t e d charges c o u l d n o t b e c o l l e c t e d .

The

d e r i v a t i z e d e l e c t r o d e surfaces m a y a l l o w t h e e x p l o i t a t i o n of s u c h effects. I n o u r studies t o date w e h a v e b e e n m a i n l y c o n c e r n e d w i t h d e r i v a t i z i n g s m a l l b a n d g a p m a t e r i a l s w i t h t h e a i m of m a n i p u l a t i n g c h a r g e transfer k i n e t i c s t o p r e v e n t p h o t o a n o d i c d e c o m p o s i t i o n of t h e s e m i c o n d u c t o r . O t h e r w o r k e r s h a v e u n d e r t a k e n t h e d e r i v a t i z a t i o n of i n e r t , b u t w i d e b a n d g a p , oxides s u c h as S n 0 a n d T i 0 2

2

w i t h visible-light-absorbing

d y e m o l e c u l e s (34-^37). T h e p a r t i c u l a r e m p h a s i s i n these systems has b e e n to sensitize t h e w i d e b a n d g a p oxides to v i s i b l e l i g h t f o r t h e H 0 2

s p l i t t i n g r e a c t i o n , b u t t h e o x i d i z e d f o r m of d y e m o l e c u l e s p r o d u c e d b y o p t i c a l e x c i t a t i o n o n t h e surface is g e n e r a l l y i n c a p a b l e of e v o l v i n g

0

2

f r o m H 0 . F u r t h e r , i n one pass of t h e l i g h t , t h i n l a y e r s of d y e m o l e c u l e s 2

are i n c a p a b l e of a b s o r b i n g a l a r g e f r a c t i o n of t h e i n c i d e n t i r r a d i a t i o n . I t is also n o t c l e a r w h e t h e r t h e efficiency of g e n e r a t i o n of s e p a r a t e d e l e c t r o n - h o l e p a i r s w i l l b e h i g h , o w i n g to t h e fact t h a t t h e e l e c t r o n - h o l e p a i r s are g e n e r a t e d i n t h e d y e m o l e c u l e a n d t r a n s f e r of a n e l e c t r o n t o t h e c o n d u c t i o n b a n d of t h e s e m i c o n d u c t o r m a y n o t b e c o m p l e t e l y effi cient. I n o u r e x p e r i m e n t s t h u s f a r t h e a i m has b e e n to d e r i v a t i z e elec trodes w i t h a r e d o x c o u p l e A*/A

t h a t is t r a n s p a r e n t to v i s i b l e l i g h t so

t h a t l i g h t a b s o r p t i o n results i n e l e c t r o n - h o l e p a i r g e n e r a t i o n i n a r e g i o n


272

INTERFACIAL

of h i g h field n e a r t h e surface of t h e s e m i c o n d u c t o r .

PHOTOPROCESSES

T h e systems w h i c h

h a v e r e c e i v e d d e t a i l e d s t u d y a t this p o i n t a r e n - t y p e G e (38) 40) h a v i n g b a n d gaps of 0.7 a n d 1.1 e V (41),

a n d S i (39,

respectively, derivatized (OEt)

SiCl

a

Fe

2


(EtO)sSi III

II w i t h t h e h y d r o l y t i c a l l y u n s t a b l e ferrocenes (l,r-ferrocenediyl)dichlorosilane

trichlorosilylferrocene

(I),

(II), a n d l , l - f o w ( t r i e t h o x y s i l y l ) f e r r o /

cene (III). F o r purposes of c o m p a r i s o n i n terms of e l e c t r o c h e m i c a l b e h a v i o r , A u (42)

a n d P t (43,44)

electrodes h a v e b e e n d e r i v a t i z e d w i t h

I, II, a n d III a n d c h a r a c t e r i z e d b y e l e c t r o a n a l y t i c a l t e c h n i q u e s . Working

Hypotheses

We

set o u t t o i l l u s t r a t e t h e p r i n c i p l e s of p h o t o e l e c t r o a c t i v i t y

of

s u r f a c e - a t t a c h e d r e d o x c o u p l e s w h e r e t h e surface is a n n - t y p e s e m i c o n ductor.

O u r w o r k i n g hypotheses center a b o u t t h e m o d e l f o r t h e ener

getics of t h e n - t y p e s e m i c o n d u c t o r - l i q u i d i n t e r f a c e (44).

F i g u r e 1 shows

a c o m p a r i s o n of t h e i n t e r f a c i a l energetics f o r a n a k e d n - t y p e

semicon

d u c t o r a n d f o r t h e same s e m i c o n d u c t o r d e r i v a t i z e d w i t h a r e d o x c o u p l e , A*/A,

where E

BG

is t h e s e m i c o n d u c t o r b a n d g a p , E is t h e f e r m i l e v e l , o r t

e l e c t r o c h e m i c a l p o t e n t i a l of t h e s e m i c o n d u c t o r , E edox (A*/A)

a n d E dox

r

re

are t h e e l e c t r o c h e m c i a l p o t e n t i a l s of a t t a c h e d a n d s o l u t i o n c o u

(B /B) +

ples, respectively, a n d £ B a n d E B are the valence-band a n d conductionV

C

b a n d p o s i t i o n s , r e s p e c t i v e l y , o n a n e l e c t r o c h e m i c a l p o t e n t i a l scale.

We

assume here a n i d e a l s i t u a t i o n w h e r e there a r e n o surface states b e t w e e n E B and E V

C

B

a n d t h e p o s i t i o n of E

V

B

a n d E B is i n d e p e n d e n t of w h e t h e r C

o r n o t t h e A / A c o u p l e is a t t a c h e d . A t c h a r g e - t r a n s f e r e q u i l i b r i u m i n t h e +

dark, E , E F

redox

and E d

(A /A),

r e

0 X

(B /B) +

m u s t a l l b e t h e same, b u t

u p o n illumination at open-circuit w i t h photons w i t h potentials than E E

F

B

B

G t h e v a l u e of E

t

, and |E

r e d 0

T h e v a l u e of E

— E

x (B /B) +

R E D O X

approaches

(A /A) +

F B

the so-called

flat-band

| represents t h e m a x i m u m

c a n be no more positive than E

greater

potential,

photovoltage. V

B

, a n d repre

sents t h e m a x i m u m o x i d i z i n g p o w e r t h a t c a n b e a c h i e v e d u n d e r i l l u m i n a t i o n . T h e p o s i t i o n of E

r e d

o x (A*/A)

is d i s c u s s e d i n m o r e d e t a i l b e l o w . A s

is o f t e n f o u n d , w e assume t h a t t h e n - t y p e s e m i c o n d u c t o r

e l e c t r o d e is



Figure

I.

LIQUID

_ J(B/_Bj_ -E^/A) VB

COUNTERELECTRODE

H§J3l. -E(A /A)

LIQUID

"VB

.

XB

LOAD

LIQUID

E(B /B) ^"E(A /A) VB

CB

COUNTERELECTRODE

(b) ILLUMINATED OPEN-CIRCUIT

SEMICONDUCTOR

i Valence Band

BG

Conduction Band

External Circuit

+

Interface energetics for an n-type semiconductor derivatized immersed in a solution of B / B

+

COUNTERELECTRODE

with the redox couple A / A and

(c) ENERGETICS FOR ILLUMINATED CELL IN OPERATION

J~ SEMICONDUCTOR

Valence Band

BG

induction Band

(a) DARK EQUILIBRIUM or ILLUMINATED SHORT-CIRCUIT

SEMICONDUCTOR

Valence Band

"BG

Conduction Band

CB

External Circuit


3

8-

o

3

ft.

>

W H

o a

3

01

274

INTERFACIAL

PHOTOPROCESSES

b l o c k i n g to o x i d a t i o n r e a c t i o n s i n t h e d a r k , e v e n f o r t h e s u r f a c e - a t t a c h e d c o u p l e . O w i n g t o t h e b a r r i e r t o e l e c t r o n transfer, r e d u c t i o n s at t h e n - t y p e s e m i c o n d u c t o r are s l u g g i s h f o r E

more positive than E

t

t h e v a l u e of E

m a y be more negative than E

t

r e d 0

x.

F

B

, even though

This rectifying junc

t i o n is s i m i l a r t o a S c h o t t k y b a r r i e r w h e r e t h e l i q u i d p l a y s t h e r o l e of a metal having E

t

E

=

( B / B ) . I n this i d e a l m o d e l , the m a x i m u m out +

r e d o x

p u t s f o r t h e n a k e d a n d d e r i v a t i z e d electrodes

a r e t h e same.

A t short

c i r c u i t u n d e r i l l u m i n a t i o n , t h e i n t e r f a c i a l energetics are t h e same as i n t h e d a r k at c h a r g e t r a n s f e r e q u i l i b r i u m , f o r fast e q u i l i b r a t i o n of A / A +

with B / B . +


W h e n a c t u a l l y o p e r a t i n g , t h e o b j e c t i v e is to m a x i m i z e t h e v a l u e of q u a n t u m y i e l d for e l e c t r o n Eredox ( B / B ) | , w h e r e

E

+

t

flow

() t i m e s t h e o u t p u t v o l t a g e | E — e

f

is m o r e n e g a t i v e t h a n E

on

(B*/B)

redox

the

e l e c t r o c h e m i c a l scale. S i n c e t h e r e m u s t b e s o m e a m o u n t of b a n d b e n d i n g t o separate e l e c t r o n - h o l e p a i r s a n d to p r e v e n t b a c k e l e c t r o n t r a n s f e r , E must be somewhat more positive than E

F

B

. N e t B —» B

+

f

c o n v e r s i o n is i n

competition w i t h direct electron-hole recombination i n the semiconductor a n d b a c k e l e c t r o n transfer to r e d u c e B b a c k to B . S e m i c o n d u c t o r - l i q u i d +

i n t e r f a c e d i a g r a m s f o r a c e l l i n o p e r a t i o n are i n c l u d e d i n F i g u r e 1. t h e d e r i v a t i z e d e l e c t r o d e , t h e v a l u e of E must be situated between E

red

ox ( A / A )

For

during operation

+

( B / B ) a n d E B , b u t the position w i l l +

r e d o x

V

d e p e n d o n t h e rate of e q u i l i b r a t i o n w i t h t h e B / B c o u p l e r e l a t i v e t o t h e +

rate of o x i d a t i o n b y p h o t o g e n e r a t e d holes. tion A

+

I t is d e s i r a b l e t h a t t h e r e a c

- f B - » B p r o c e e d s at a rate faster t h a n t h a t f o r t h e h o l e o x i d a t i o n +

of B f o r t h e n a k e d s e m i c o n d u c t o r case.

B u t the ultimate advantages a n d

u t i l i t y d o n o t n e c e s s a r i l y d e p e n d o n t h i s p r o p e r t y , since m o l e c u l a r s p e c i ficity,

f o r e x a m p l e , n e e d n o t i n v o l v e fast rates.

Rationale

for

Choice of Systems Studied

S e v e r a l factors

governed

o u r c h o i c e of i n i t i a l systems

F i r s t , studies i n t h i s l a b o r a t o r y (12)

for

study.

e s t a b l i s h e d t h a t f e r r o c e n e is c a p a b l e

of c a p t u r i n g p h o t o g e n e r a t e d holes a t n - t y p e S i a t a rate t h a t w o u l d p r e c l u d e p h o t o a n o d i c surface r e a c t i o n t o p r o d u c e i n s u l a t i n g SiO^. l a y e r s i n a n o n a q u e o u s e l e c t r o l y t e s o l u t i o n . F u r t h e r , t h e surface of S i bears f u n c t i o n a l g r o u p s t h a t a l l o w c o v a l e n t a t t a c h m e n t of substances s u c h as I, II, A

surface—OH +

R^RaSi—CI

surface—OH +

R R R Si—0R

surface—0—SiRiR R + 2

3

HC1

(1)

A 1

2

3

4

-> s u r f a c e — 0 — S i R i R R + 2

3

R 0H 4

(2)

o r III b y t h e g e n e r a l r e a c t i o n i n d i c a t e d i n E q u a t i o n s 1 o r 2. S u c h surface d e r i v a t i z a t i o n c h e m i s t r y has a m p l e p r e c e d e n c e i n a n u m b e r of areas ( 4 5 , 46,47)

i n c l u d i n g d e r i v a t i z a t i o n of

reversible

electrodes

(48,49,50).


15.

WRIGHTON E T AL.

Derivatized

Semiconductor

Photoelectrodes

275

T h u s , w e have c o m b i n e d the general chemistry represented i n E q u a t i o n s 1 a n d 2 w i t h t h e finding t h a t f e r r o c e n e n e u t r a l i z e s holes at S i at a fast rate ( 1 2 ) .

W e also b e g a n w i t h t h e k n o w l e d g e t h a t s i m p l e d e r i v a t i v e s

of f e r r o c e n e a n d f e r r o c e n e i t s e l f d o n o t h a v e s u b s t a n t i a l l y different r e d o x properties

(kinetics a n d energetics)

(51).

Examining derivatized G e

(38)

w a s a n o u t g r o w t h of t h e w o r k o n S i (49,40),

(42)

a n d P t (42,43)

a n d the work on A u

surfaces w a s u n d e r t a k e n t o e s t a b l i s h s o m e p o i n t s

of reference w i t h respect to energetics a n d k i n e t i c s of e l e c t r o n t r a n s f e r of a t t a c h e d f o r m s of f e r r o c e n e

f r o m r e a c t i o n of I, II, o r III w i t h

the

f u n c t i o n a l i z e d surface.


T h e systems t h a t are d i s c u s s e d h e r e a r e p r o t o t y p i c ; a n u m b e r

of

different systems c a n n o w b e e n v i s i o n e d a n d w o r k is u n d e r w a y to e l a b o rate t h e c o n c e p t s d e s c r i b e d h e r e i n . O u r a i m has b e e n to i l l u s t r a t e s o m e n e w t e c h n i q u e s f o r p o t e n t i a l u t i l i z a t i o n , c o n v e r s i o n , a n d storage of o p t i cal

energy.

Results

and

Discussion R e a c t i o n of p r e t r e a t e d

Derivatized A u and Pt Electrodes (42,43). (anodized)

A u or P t e l e c t r o d e surfaces w i t h isooctane s o l u t i o n s of I , II,

or III at r o o m t e m p e r a t u r e results i n t h e a t t a c h m e n t of material.

electroactive

T h i s is d e t e r m i n e d b y c y c l i c v o l t a m m e t r y of t h e d e r i v a t i z e d

electrodes i n n o n a q u e o u s e l e c t r o l y t e solutions c o n t a i n i n g n o d e l i b e r a t e l y a d d e d electroactive materials. Some representative electroanalytical data (42,43)

are i n c l u d e d i n T a b l e I, a n d t h e c y c l i c v o l t a m m e t r i c scans i n

F i g u r e 2 are t y p i c a l . T h e e s s e n t i a l findings a r e as f o l l o w s : t h e e l e c t r o a c t i v e m a t e r i a l is l i k e l y o l i g o m e r i c , e s s e n t i a l l y r e v e r s i b l y e l e c t r o a c t i v e , a n d p e r s i s t e n t l y a t t a c h e d ; i n m o s t respects, t h e p r o p e r t i e s of t h e d e r i v a t i z e d surfaces are as e x p e c t e d Table I.

(52)

for a surface-attached, reversible,

Anodic Peak Positions for Derivatized Electrodes

Electrode Material

Derivatizing Reagent

Pt° Pt Pt° Au° n-type Ge* n-type Ge* n-type S i * n-type S i *

I II III II I II I II

a

EiPA CV vs.

0.53 0.51 0.60 0.47 -0.3 -0.3 -0.1 -0.05

(avg. (avg. (avg. (avg.

of of of of

SCE)

7 electrodes) 10 electrodes) 7 electrodes) 20 electrodes)

" M e t a l electrodes are for C H C N / 0 . 1 M [ n - B u N ] C 1 0 electrolyte solution. Cyclic voltammetry reveals reversible behavior. F o r semiconductor electrodes the most negative photoanodic peaks are given for E t O H / O . l M " n - B u N ] C 1 0 electrolyte solution. 3

4

4

b

4

4


INTERFACIAL

276

PHOTOPROCESSES


T

I

I

I

I

I

I

-0.2

Q0

+0.2

+0.4

+0.6

Potential, V vs

i

I

+0.8

SCE

Figure 2. Cyclic voltammograms for Au derivatized with 11 as a function of scan rate in 0.1 M [n-Bu N]Cl0 in CH CN at 298 K. Coverage of electroactive material is 6.2 X 10~ mol/cm . The inset shows a plot of peak anodic current against scan rate. 4

4

S

9

2

o n e - e l e c t r o n r e d o x c o u p l e . C y c l i c v o l t a m m e t r y reveals t h a t e l e c t r o a c t i v e surfaces r e m a i n e s s e n t i a l l y u n c h a n g e d f o r e i g h t w e e k s of shelf storage a n d c a n be c y c l e d between oxidized a n d reduced f o r m thousands t i m e s w i t h o u t d e t e r i o r a t i o n . F o r t y p i c a l electrodes t h e E°

of

for the at

t a c h e d m a t e r i a l is w i t h i n 100 m V of t h e v a l u e ( 5 1 ) f o r E ° ( f e r r i c e n i u m / f e r r o c e n e ) i n s o l u t i o n , as has b e e n g e n e r a l l y f o u n d f o r o t h e r d e r i v a t i z e d


15.

WRIGHTON E T AL.

electrodes.

Derivatized

Semiconductor

277

Photoelectrodes

G e n e r a l l y t h e p e a k c u r r e n t is d i r e c t l y p r o p o r t i o n a l t o s c a n

rate, at least u p t o 200 m V / s e c . T h o u g h t h e s e p a r a t i o n of t h e a n o d i c a n d c a t h o d i c c u r r e n t p e a k s g e n e r a l l y increases at fast s c a n rates, t h e p e a k - t o p e a k s e p a r a t i o n c a n b e n e a r l y z e r o a n d w e l l b e l o w 60 m V f o r s c a n rates as h i g h as 500 m V / s e c . T h e r e is at least one p r o p e r t y of t h e d e r i v a t i z e d P t a n d A u electrodes t h a t is n o t i n a c c o r d w i t h t h e n o t i o n of a r e v e r s i b l e , o n e - e l e c t r o n r e d o x c o u p l e b o u n d to t h e surface of a r e v e r s i b l e e l e c t r o d e — t h e f u l l w i d t h of t h e c y c l i c v o l t a m m e t r i c w a v e s at h a l f h e i g h t is t y p i c a l l y i n t h e r a n g e of 2 0 0 - 3 0 0 m V r a t h e r t h a n t h e 90 m V t h e o r e t i c a l l y e x p e c t e d ( 5 2 ) .

Values


n e a r t h e t h e o r e t i c a l v a l u e of 90 m V h a v e b e e n o b t a i n e d i n other systems; i n p a r t i c u l a r , p o l y v i n y l f e r r o c e n e o n P t gives c y c l i c v o l t a m m e t r i c w a v e s t h a t are c o n s i d e r a b l y s h a r p e r ( 5 3 ) t h a n those w e h a v e f o u n d for d e r i v a t i z a t i o n of A u a n d P t w i t h I, II, a n d II. P t a n d A u electrodes d e r i v a t i z e d w i t h II h a v e b e e n s u b j e c t e d analysis b y e l e c t r o n s p e c t r o s c o p y (54);

to

these results a n d those f r o m t h e

e l e c t r o a n a l y t i c a l c h a r a c t e r i z a t i o n are i n a c c o r d w i t h a surface t h a t has o l i g o m e r s of t h e e l e c t r o a c t i v e ferrocene u n i t s l i n k e d b y - S i - O - S i - b o n d s . T h e e l e c t r o n spectroscopy of d e r i v a t i z e d surfaces a n d e l e m e n t a l analyses of substances f o r m e d b y e x p o s i n g II to a i r a n d m o i s t u r e i n d i c a t e t h a t t h e ferrocene u n i t s d o not r e m a i n c o m p l e t e l y i n t a c t u p o n r e a c t i o n . T h e d a t a i n d i c a t e loss of i r o n f r o m t h e m a t e r i a l , a fact n o t i n c o n s i s t e n t w i t h k n o w n s u s c e p t i b i l i t y of ferrocenes t o d e c o m p o s i t i o n i n n o n a q u e o u s solutions c o n taining nucleophiles (55).

Nonetheless, the cyclic voltammetry demands

t h e presence of e l e c t r o a c t i v e m a t e r i a l p e r s i s t e n t l y a t t a c h e d to t h e surface. T h e loss of i r o n a n d t h e v a r i o u s p o s s i b l e o l i g o m e r i c structures l i k e l y cause the rather b r o a d cyclic voltammetric waves on A u a n d Pt. W e

have

a d o p t e d t h e i n t e r p r e t a t i o n t h a t t h e r e is a v a r i e t y of ferrocene u n i t s o n t h e surface, essentially i n d e p e n d e n t of surface c o v e r a g e , e a c h w i t h its o w n E ° . Derivatized w-Type Si (39,40).

R e a c t i o n of s i n g l e c r y s t a l S i a n d

G e surfaces w i t h I, II, a n d III results i n t h e persistent a t t a c h m e n t of e l e c t r o a c t i v e m a t e r i a l . T a b l e I i n c l u d e s some e l e c t r o a n a l y t i c a l d a t a f o r these d e r i v a t i z e d surfaces.

F i g u r e s 3, 4, a n d 5 s h o w a r e p r e s e n t a t i v e

e l e c t r o a n a l y t i c a l c h a r a c t e r i z a t i o n of a d e r i v a t i z e d n - t y p e S i e l e c t r o d e ; F i g u r e 6 shows t h e c o m p a r a b l e d a t a f o r a n a k e d n - t y p e S i e l e c t r o d e i n t h e same e l e c t r o l y t e s o l u t i o n . T h e c y c l i c v o l t a m m o g r a m s of t h e n a k e d elec trode i n the electrolyte solution illustrate the p r o b l e m w i t h n-type photo a n o d e s ; t h e p h o t o a n o d i c c u r r e n t c o r r e s p o n d s to t h e g r o w t h of a n i n s u l a t i n g l a y e r of o x i d e m a t e r i a l ( S i O * ) t h a t c a n n o t b e r e d u c e d o v e r t h e p o t e n t i a l r a n g e s c a n n e d . I t is this r e a c t i o n t h a t m u s t b e s u p p r e s s e d i n t h e use of n - t y p e S i as a p h o t o a n o d e .

T h e solvent i n F i g u r e 6 is C H C N 3

a n d t h e source of t h e o x i d e o x y g e n is t r a c e H 0 i n s o l u t i o n . A l s o , n a k e d 2


INTERFACIAL


278

Potential , V vs

PHOTOPROCESSES

SCE

Figure 3. Cyclic voltammograms for derivatized n-type Si showing tative effects of light intensity. "Mono-Claw" is reagent I.

quali

S i bears a n o x i d e l a y e r of some thickness t h a t a p p a r e n t l y does n o t p r e c l u d e e l e c t r o n transfer. U n d e r t h e c o n d i t i o n s s h o w n , less t h a n 10" C / c m 2

2

effectively passivates the surface to the flow of p h o t o a n o d i c c u r r e n t . C o n s e q u e n t l y , p r o t e c t i o n of n - t y p e

S i f r o m surface p h o t o a n o d i c

reaction

d e p e n d s o n e x t r e m e l y c o m p e t i t i v e h o l e c a p t u r e processes. F o r e x a m p l e , a s s u m i n g t h a t a m b i e n t solar i n t e n s i t y w o u l d y i e l d a b o u t 40 m A / c m c u r r e n t d e n s i t y , a h o l e c a p t u r e process t h a t w a s o n l y 9 9 . 9 9 %

2

of

efficient

c o u l d s t i l l l e a d t o S i O * f o r m a t i o n at a rate t h a t w o u l d r e n d e r t h e c e l l u s e less i n a p p r o x i m a t e l y 1 h o u r . T h e c y c l i c v o l t a m m e t r y d a t a i n c l u d e d i n F i g u r e s 3, 4, a n d 5 c a n b e r e p e a t e d a n u m b e r of times w i t h o u t significant v a r i a t i o n i n t h e essential p r o p e r t i e s , e v i d e n c i n g p r o t e c t i o n f r o m gross o x i d e g r o w t h f o u n d f o r t h e n a k e d electrode u n d e r t h e same c o n d i t i o n s . W e w i l l a m p l i f y this p o i n t below. D e r i v a t i z e d n - t y p e S i e x h i b i t s l i t t l e or n o a n o d i c c u r r e n t i n t h e d a r k , b u t i l l u m i n a t i o n w i t h l i g h t of greater t h a n £ G results i n t h e B

flow

of

a n o d i c c u r r e n t . T h e c a t h o d i c r e t u r n p e a k is o b s e r v e d w h e t h e r the l i g h t is o n o r not, b u t the c a t h o d i c c u r r e n t p e a k p o s i t i o n is l i g h t d e p e n d e n t , since t h e net c u r r e n t flow w h e n t h e l i g h t is o n is t h e s u m of d a r k c a t h o d i c p l u s p h o t o a n o d i c c u r r e n t . A t a sufficiently h i g h l i g h t i n t e n s i t y , t h e p e a k a n o d i c c u r r e n t is d i r e c t l y p r o p o r t i o n a l to s c a n rate, as e x p e c t e d f o r


a

Derivatized

WRIGHTON E T A L .

15.

Semiconductor

Photoelectrodes

279

surface-attached redox couple, whereas the peak current varies w i t h the s q u a r e root of t h e s c a n rate f o r a n a k e d S i e l e c t r o d e i l l u m i n a t e d i n a n o n a q u e o u s e l e c t r o l y t e s o l u t i o n c o n t a i n i n g ferrocene. T h e peak potentials c a n be substantially more negative than E ° (ferricenium/ferrocene)

f o r i l l u m i n a t e d n - t y p e S i , i n contrast t o t h e

s i t u a t i o n f o r P t o r A u . T h e p e a k p o t e n t i a l s c o r r e s p o n d c l o s e l y t o those f o r n a k e d n - t y p e S i i n a n o n a q u e o u s e l e c t r o l y t e s o l u t i o n of ferrocene. T h e extent t o w h i c h t h e s u r f a c e - a t t a c h e d ferrocene

material can be

o x i d i z e d a t a p o t e n t i a l m o r e n e g a t i v e t h a n o n a r e v e r s i b l e e l e c t r o d e is a m e a s u r e of t h e o u t p u t v o l t a g e f o r a c e l l u s i n g s u c h a p h o t o e l e c t r o d e .

If


t h e p h o t o a n o d i c c u r r e n t p e a k is s y m m e t r i c a l , t h e p e a k p o t e n t i a l r e p r e sents t h e p o t e n t i a l a t w h i c h t h e s u r f a c e - a t t a c h e d m a t e r i a l is 5 0 % o x i d i z e d a n d is thus t h e e l e c t r o d e p o t e n t i a l , E , at w h i c h Eredox f o r t h e f

s u r f a c e - a t t a c h e d species is e q u a l t o t h e f o r m a l p o t e n t i a l , E ° . W e h a v e o b s e r v e d p h o t o a n o d i c p e a k p o t e n t i a l s f o r d e r i v a t i z e d n - t y p e S i as n e g a t i v e as a p p r o x i m a t e l y —0.1 V vs. S C E , b u t t h e v a l u e is t y p i c a l l y a r o u n d + 0 . 1 V vs. S C E . A n o d i c peak potentials for derivatized P t a n d A u are i n t h e r a n g e + 0 . 4 - 0 . 6 V vs. S C E . T h u s , t h e o u t p u t v o l t a g e f o r d e r i v a t i z e d n - t y p e S i p h o t o e l e c t r o d e s is i n t h e r a n g e 3 0 0 - 7 0 0 m V , a s s u m i n g t h a t t h e t h e r m o d y n a m i c s f o r t h e v a r i o u s s u r f a c e - a t t a c h e d substances a r e i n d e p e n d e n t of t h e surface. T h i s a s s u m p t i o n s h o u l d b e v a l i d w i t h i n 100 m V (56).

i

1

1

r

n-Type Si lOOmV/sec

-0.4

0.0 Potential, V vs SCE

+0.4

Figure 4. As in Figure 3 except light is turned off at the anodic limit (+ 0.6 V) to show that the cathodic peak does not require illumination; ( ) result obtained when the light is left on for the entire scan.


280

INTERFACIAL


i

i

r

Potential, Figure 5.

PHOTOPROCESSES

V vs SCE

Scan rate dependence of cyclic voltammograms for same elec trode and electrolyte solution as in Figures 3 and 4.

1

1

1

n - Type S i , lOOmV/sec 632.8nm Illumination C H C N , 0.1 M C n - B u N ] C I 0 3

4

i

i

1st Scan

4

/ /

T IO/XA

y

1

^^^^L

2nd Scan 3rd Scan

t o - O

i -0.8

i -0.4

i 0.0 Potential, V vs

I

I

+0.4 SCE

+0.8

Figure 6. Cyclic voltammograms for naked n-type Si in same electro lyte and under same illumination conditions as for derivatized electrode in Figure 5. Note the lack of a cathodic wave and the declining photo anodic current on the successive scans reflecting SiO growth. a


15.

WRIGHTON E T AL.

Photoelectrodes

281

T h e energetics e s t a b l i s h e d a b o v e c o r r e s p o n d w e l l w i t h those

found

Derivatized

Semiconductor

for a n a k e d n-type S i exposed to a nonaqueous ferricenium/ferrocene.

e l e c t r o l y t e s o l u t i o n of

A t least f o r t h i s s e m i c o n d u c t o r

t h e n , t h e c o v a l e n t a t t a c h m e n t of t h e e l e c t r o a c t i v e

photoelectrode,

m a t e r i a l does

not

s i g n i f i c a n t l y a l t e r t h e i n t e r f a c e energetics, as is g e n e r a l l y t r u e f o r r e v e r s i b l e e l e c t r o d e systems ( 5 6 ) .

T h e p o t e n t i a l onset f o r p h o t o a n o d i c c u r r e n t

at t h e h i g h e s t l i g h t i n t e n s i t y is a reasonable a p p r o x i m a t i o n of E

F

B

; we

o f t e n find t h a t t h e c a t h o d i c c u r r e n t p e a k i n t h e d a r k is s u b s t a n t i a l l y more positive than E

F

B

. T h i s o b s e r v a t i o n i n other systems

(25,26,27)

has b e e n t a k e n to i n d i c a t e t h a t t h e r e are i n t e r f a c e or surface states of t h e


s e m i c o n d u c t o r t h a t c a n b e filled w i t h electrons at a p o t e n t i a l m o r e p o s i tive than E

F

B

. W e a d o p t t h i s i n t e r p r e t a t i o n a n d note t h a t t h e c r u c i a l

i n t e r f a c e states seem to b e present for b o t h n a k e d a n d d e r i v a t i z e d S i . This

finding

is consistent w i t h t h e c o n c l u s i o n t h a t s u c h i n t e r f a c e states

are l i k e l y associated w i t h t h e S i O j . l a y e r t h a t is i n v a r i a b l y present o n S i a n d o n w h i c h t h e d e r i v a t i z i n g l a y e r is b u i l t . I d e a l l y , s u c h i n t e r f a c e states c o u l d b e m a n i p u l a t e d to p r e c l u d e r e d u c t i o n of f e r r i c e n i u m at p o t e n t i a l s more positive than E

F

B

; photoanodic a n d cathodic peaks w o u l d be situ

a t e d m o r e or less s y m m e t r i c a l l y a b o u t E

F

B

. T h e i m p o r t a n c e of a n o x i d e

l a y e r b e t w e e n n - t y p e G a A s a n d A u is e v i d e n t i n a S c h o t t k y b a r r i e r solar c e l l , w h e r e significant changes i n solar efficiency w e r e f o u n d w i t h v a r i a tion i n the oxide layer ( 5 7 ) . T h e shape of t h e c y c l i c v o l t a m m e t r i c w a v e s f o r d e r i v a t i z e d S i is q u i t e v a r i a b l e , d e p e n d i n g o n t h e exact p r o c e d u r e for d e r i v a t i z a t i o n , s c a n rate, a n d l i g h t i n t e n s i t y . P e a k w i d t h s at h a l f h e i g h t as n a r r o w as 110 m V h a v e b e e n o b s e r v e d , b u t m o r e t y p i c a l v a l u e s are i n a r a n g e s i m i l a r t o that for derivatized P t a n d A u .

T h e peak potentials vary w i t h

scan

rate, d e p e n d i n g o n c o v e r a g e a n d l i g h t i n t e n s i t y ; h i g h e r coverages a n d l o w e r l i g h t i n t e n s i t y y i e l d p h o t o a n o d i c c u r r e n t peaks t h a t are m o r e a n o d i c at t h e faster scan rates. T h e d a r k c u r r e n t - p o t e n t i a l p r o p e r t i e s are also quite variable, depending on the preparation procedure; generally there is n o significant d a r k a n o d i c c u r r e n t for p o t e n t i a l s m o r e n e g a t i v e t h a n about

+0.6 V .

S o m e d e r i v a t i z e d electrodes

have been prepared that

e x h i b i t n o d a r k o x i d a t i o n c u r r e n t at p o t e n t i a l s as p o s i t i v e as + 1 0 . 0 V vs. S C E . B u t t y p i c a l l y , s u r f a c e - a t t a c h e d ferrocenes c a n b e o x i d i z e d i n t h e d a r k at p o t e n t i a l s of a r o u n d + 1 . 0 V vs. S C E , s t i l l s u b s t a n t i a l l y m o r e a n o d i c t h a n for P t or A u . C l e a r l y , d e r i v a t i z a t i o n of n - t y p e S i c a n p r o t e c t t h e surface f r o m s i g nificant p h o t o a n o d i c

SiO

x

formation.

T h i s c o n c l u s i o n is b a s e d o n t h e

o b s e r v a t i o n t h a t t h e c y c l i c v o l t a m m e t r i c w a v e s for the i l l u m i n a t e d d e r i v a t i z e d surface are essentially u n c h a n g e d for m a n y scans. I f the S i O * l a y e r were g r o w i n g significantly the photoanodic peak w o u l d be more anodic w i t h the t h i c k e r o x i d e l a y e r , w h i l e t h e c a t h o d i c p e a k w o u l d b e negative.

T h e s e changes

are o b s e r v e d

for p r o l o n g e d

more

c y c l i n g a n d are


282

INTERFACIAL

PHOTOPROCESSES

p a r t i c u l a r l y gross at v e r y a n o d i c p o t e n t i a l s u n d e r i l l u m i n a t i o n .

These

observations suggest t h a t t h e o x i d e g r o w t h rate is, n o t u n e x p e c t e d l y , a f u n c t i o n of b o t h l i g h t i n t e n s i t y a n d p o t e n t i a l . T h e i m p o r t a n t finding is t h a t s u b s t a n t i a l l y m o r e t h a n 10" C / c m 2

2

of p h o t o a n o d i c c u r r e n t c a n pass

t h r o u g h t h e d e r i v a t i z e d e l e c t r o d e i n t e r f a c e w i t h o u t p a s s i v a t i n g t h e elec t r o d e . L e s s t h a n 10" C / c m 2

2

passivates t h e n a k e d electrode.

Protection f r o m S i O * g r o w t h d u r i n g c e l l operation w i l l be discussed b e l o w , b u t t h e r a t i o n a l e f o r t h e d u r a b i l i t y of the d e r i v a t i z e d surface w i l l b e m e n t i o n e d here. P h o t o g e n e r a t e d holes i n the v a l e n c e b a n d of S i are t r a n s f e r r e d r a p i d l y t o the s u r f a c e - a t t a c h e d ferrocene centers. T h e rate of


transfer to t h e ferrocenes is a p p a r e n t l y fast e n o u g h t o p r e c l u d e p r o m p t o x i d e f o r m a t i o n . W h e n t h e l i g h t i n t e n s i t y is l o w e n o u g h a n d the c o v e r a g e of ferrocene great e n o u g h t h i s r a p i d h o l e transfer t o f o r m f e r r i c e n i u m centers c a n b e r e g a r d e d as essentially i r r e v e r s i b l e , since E more positive than E

r e d o x

V

B is m u c h

f o r t h e a t t a c h e d species w h e n there is l o w

f r a c t i o n a l c o n v e r s i o n of ferrocene to f e r r i c e n i u m . B u t f o r situations w h e r e Eredox f o r the s u r f a c e - a t t a c h e d species is p o s i t i v e e n o u g h , there m a y b e a significant steady-state h o l e c o n c e n t r a t i o n o n the S i . F o r this s i t u a t i o n w e c o n c l u d e t h a t o x i d e g r o w t h is k i n e t i c a l l y i n h i b i t e d b y the i n a c c e s s i b i l i t y of the holes to attack b y H 0 . T h u s , t h e o l i g o m e r i c ferrocene l a y e r 2

p h y s i c a l l y protects t h e u n d e r l y i n g S i / S i O * surface, as w e l l as p r o v i d i n g a s y s t e m t h a t c a n r a p i d l y a c c e p t p h o t o g e n e r a t e d holes. I n the l i m i t of v e r y l a r g e , p o l y m e r i c coverages or at sufficiently l o w l i g h t i n t e n s i t y , t h e d e r i v a t i z e d n - t y p e S i electrode behaves l i k e a n a k e d e l e c t r o d e exposed to a n e l e c t r o l y t e s o l u t i o n t h a t c o n t a i n s a r e d u c t a n t c a p a b l e of i r r e v e r s i b l y a n d r a p i d l y c a p t u r i n g every p h o t o g e n e r a t e d h o l e . F o r l o w coverages of s u r f a c e - a t t a c h e d m a t e r i a l w e t y p i c a l l y o b s e r v e " b r e a k i n " changes i n t h e c y c l i c v o l t a m m e t r y t h a t c o r r e s p o n d to o x i d e g r o w t h a n d p a s s i v a t i o n i n areas of t h e surface w h e r e t h e r e is n o , o r l o w , c o v e r a g e . T h e first f e w scans s h o w l a r g e p h o t o a n o d i c c u r r e n t s , b u t t h e r e is n o s i m i l a r a m o u n t of c a t h o d i c c u r r e n t ; t h e a m o u n t of s u c h p h o t o a n o d i c c u r r e n t d e c l i n e s w i t h e a c h successive scan u n t i l the c u r r e n t - v o l t a g e p r o p erties b e c o m e essentially constant a n d t h e i n t e g r a t e d p h o t o a n o d i c

and

c a t h o d i c c u r r e n t s are e q u a l . T h e l a r g e p h o t o a n o d i c c u r r e n t l i k e l y c o r r e sponds t o SiO

x

g r o w t h o n n o n d e r i v a t i z e d areas of t h e electrode surface.

T h e f r a c t i o n of t h e surface t h a t is n o n d e r i v a t i z e d is, of course, v a r i a b l e b u t c a n b e i n s i g n i f i c a n t as d e t e r m i n e d b y t h e r e l a t i v e c u r r e n t d e n s i t y at n a k e d a n d d e r i v a t i z e d electrodes.

T h e oxide that grows on the n o n

d e r i v a t i z e d e l e c t r o d e areas does n o t s i g n i f i c a n t l y a l t e r t h e p r o p e r t i e s of t h e a t t a c h e d e l e c t r o a c t i v e m a t e r i a l ; t h e a m o u n t of m a t e r i a l a n d t h e p e a k p o t e n t i a l s are r e l a t i v e l y constant d u r i n g t h e b r e a k - i n of a g o o d e l e c t r o d e as d e t e r m i n e d b y t h e p o s i t i o n a n d a r e a u n d e r the c a t h o d i c peak. P e r s i s t e n t a t t a c h m e n t of e l e c t r o a c t i v e

material a n d constancy

of

energetics f o r t h e d e r i v a t i z e d n - t y p e S i h a v e b e e n i l l u s t r a t e d i n s e v e r a l


15.

WRIGHTON E T AL.

Derivatized

Semiconductor

Photoelectrodes

283

w a y s . F i r s t , d e r i v a t i z e d electrodes h a v e g o o d shelf l i f e ; t h e y h a v e b e e n stored for weeks a n d have still exhibited photoelectroactivity associated w i t h a n a t t a c h e d ferrocene d e r i v a t i v e . S e c o n d , i l l u m i n a t e d , d e r i v a t i z e d electrodes c a n b e c y c l e d a t l O O m V / s e c b e t w e e n p o t e n t i a l l i m i t s c o r r e s p o n d i n g t o c y c l i c a l o x i d a t i o n a n d r e d u c t i o n of t h e a t t a c h e d m a t e r i a l . T h e a t t a c h e d e l e c t r o a c t i v e m a t e r i a l is lost s l o w l y ( h u n d r e d s of c y c l e s ) , e v i d e n c e d b y d e c l i n i n g i n t e g r a t e d p e a k areas, w i t h o u t s i g n i f i c a n t s h i f t i n t h e p e a k p o s i t i o n s . G e n e r a l l y , t h o u g h , t h e effect of p r o l o n g e d u s e i s a shift of the photoanodic peak to more positive potentials a n d of t h e c a t h o d i c p e a k t o m o r e n e g a t i v e p o t e n t i a l s . T h e p e a k s h i f t i n g is p r o b a b l y


a c o n s e q u e n c e o f t h e g r o w t h of some u n d e r m i n i n g S i O * l a y e r , w h i l e t h e s i m p l e loss of e l e c t r o a c t i v e m a t e r i a l is l i k e l y d u e t o d e c o m p o s i t i o n o f t h e ferricenium form.

T h e d u r a b i l i t y of t h e surface is also i l l u s t r a t e d b y

c h o p p i n g the excitation b e a m w h i l e h o l d i n g the d e r i v a t i z e d electrode at a potential where photoanodic current a n d dark cathodic current occur. F i g u r e 7 illustrates t h e results f o r a c h o p p i n g f r e q u e n c y o f a b o u t 1 H z . W h i l e m u c h information can be gleaned from such data, the main point h e r e is t h a t response of t h e d e r i v a t i z e d e l e c t r o d e is e s s e n t i a l l y t h e same f o r a l a r g e n u m b e r o f cycles w h e r e there is e s s e n t i a l l y c o m p l e t e o x i d a t i o n of a l l s u r f a c e - a t t a c h e d e l e c t r o a c t i v e m a t e r i a l w h e n t h e l i g h t is t u r n e d o n . A l l d i s c u s s i o n thus f a r has c o n c e r n e d results f o r d e r i v a t i z e d S i c h a r a c t e r i z e d i n n o n a q u e o u s solutions. T h e d e r i v a t i z e d electrodes are d u r a b l e i n a q u e o u s e l e c t r o l y t e solutions as w e l l , since persistent c y c l i c v o l t a m m e t r i c w a v e s c a n b e o b s e r v e d i n aqueous e l e c t r o l y t e solutions. A l k a l i n e

i

J

\ -0.8

1

I

1

I

-0.4

r 0.5^A

i

i

.Light

N_ight J I 0.0 +0.4 Potential, V vs SCE L

12

r

L

16 Time, sec

Figure 7. Cyclic voltammograms for n-type Si derivatized with 11 (top) and current at + 0.35 V vs. SCE against time while chopping the illumination source at ~1 Hz in CH CN solution of 0.1M [n-Bu^JClO^. 3


284

INTERFACIAL

PHOTOPROCESSES

1 1

r


l

Figure 8. Cyclic voltammograms for naked n-type Ge in the presence of 2 X 10~'M ferrocene (top) and for n-type Ge derivatized with I in absence of any electroactive solu tion species. Electrolyte solution is CH CN/0.1M [n-Bu N]ClQ . 3

4

4

-0.2

0.0

+0.2

Potential, V vs

+0.4 SCE


+0.6

15.

WRIGHTON E T AL.

Derivatized

Semiconductor

285

Photoelectrodes

solutions are a v o i d e d o w i n g to t h e k n o w n s e n s i t i v i t y of f e r r i c e n i u m t o basic media (55). electrode

T h e persistent p h o t o e l e c t r o a c t i v i t y of t h e d e r i v a t i z e d

i n aqueous

s o l u t i o n is p a r t i c u l a r l y n o t e w o r t h y since H 0

is

2

v e r y l i k e l y t h e source of the o x y g e n i n S i O * f o r m a t i o n . Derivatized » - T y p e Ge ( 3 7 ) .

n-Type G e can be derivatized i n a

m a n n e r s i m i l a r to t h a t f o r n - t y p e S i . B u t u n l i k e S i , t h e d e r i v a t i z e d n - t y p e G e surfaces t h a t w e h a v e s t u d i e d e x h i b i t sufficient d a r k currents t h a t g o o d c y c l i c v o l t a m m e t r i c w a v e s for a t t a c h e d ferrocene c a n b e

observed

i n t h e d a r k . I l l u m i n a t i o n results i n a m o r e n e g a t i v e a n o d i c c u r r e n t onset, a n d t h e p e a k of t h e p h o t o a n o d i c w a v e c a n b e s h i f t e d b y u p to a r o u n d Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch015

200 m V , r e p r e s e n t i n g t h e m a x i m u m o u t p u t p h o t o v o l t a g e derivatized n-type G e .

attainable for

T h i s v a l u e accords w e l l w i t h t h e v a l u e of E

F

B

f o u n d f r o m the m a x i m u m o p e n - c i r c u i t p h o t o v o l t a g e f o r a n a k e d n - t y p e G e e l e c t r o d e e x p o s e d to a s o l u t i o n of f e r r i c e n i u m / f e r r o c e n e .

F u r t h e r , the

n a k e d n - t y p e G e exhibits d a r k a n o d i c c u r r e n t i n the presence of ferrocene w i t h a c y c l i c v o l t a m m e t r i c p e a k n e a r l y the same as t h a t f o r t h e d e r i v a t i z e d electrode ( F i g u r e 8 ) . T h u s , as for S i , d e r i v a t i z a t i o n does not a p p e a r to a p p r e c i a b l y a l t e r i n t e r f a c e energetics or k i n e t i c s f o r ferrocene

oxida

t i o n a n d the d u r a b i l i t y of d e r i v a t i z e d n - t y p e G e is s i m i l a r to t h a t f o r n-type S i . T h e o b s e r v a t i o n of significant d a r k a n o d i c c u r r e n t at n - t y p e G e l i k e l y reflects the presence of a h i g h d e n s i t y of surface states at t h e G e / G e O * i n t e r f a c e t h a t are n o t i n f l u e n c e d b y the a t t a c h m e n t of ferrocene d e r i v a tives t o t h e surface oxide. W h i l e ferrocene i n s o l u t i o n protects n - t y p e G e f r o m deleterious o x i d e f o r m a t i o n , the l o w o u t p u t v o l t a g e p r e c l u d e s a n efficient o p t i c a l e n e r g y c o n v e r s i o n d e v i c e ; w e h a v e a c c o r d i n g l y

concen

t r a t e d m o r e effort o n the use of d e r i v a t i z e d n - t y p e S i t o i l l u s t r a t e t h e p o t e n t i a l u t i l i t y of m o l e c u l a r l y m o d i f i e d photoelectrodes

i n energy

con

v e r s i o n experiments. Proof of Mediated Electron Transfer Using Derivatized if-Type Si. O n e of t h e u n i q u e features of a d e r i v a t i z e d s e m i c o n d u c t o r

photoelectrode,

c o m p a r e d w i t h a d e r i v a t i z e d m e t a l electrode, is t h a t t h e r a t i o of o x i d i z e d to reduced material depends on b o t h light a n d potential. B y definition, t h e r a t i o of o x i d i z e d t o r e d u c e d m a t e r i a l o n t h e surface of a r e v e r s i b l e electrode d e p e n d s

o n l y o n p o t e n t i a l . T h e t w o - s t i m u l i response

of

the

s e m i c o n d u c t o r d e p e n d s o n its r e c t i f y i n g p r o p e r t y a n d a l l o w s us to o b t a i n d i r e c t e v i d e n c e for m e d i a t e d o x i d a t i o n of s o l u t i o n r e d u c t a n t s . B y m e d i a t e d w e m e a n t h a t t h e o x i d a t i o n of t h e s o l u t i o n species p r o c e e d s e l e c t r o n transfer t o a h o l e l o c a l i z e d o n t h e s u r f a c e - a t t a c h e d

by

molecule.

I n t h e case of t h e s e m i c o n d u c t o r , t h e h o l e is g e n e r a t e d b y p h o t o e x c i t a t i o n a n d transferred f r o m the valence b a n d to the surface-attached


species.

286

INTERFACIAL

PHOTOPROCESSES

M e d i a t e d o x i d a t i o n is i n contrast to a d i r e c t e l e c t r o n transfer t o a h o l e l o c a l i z e d o n t h e e l e c t r o d e s u r f a c e ; w h e n t h e e l e c t r o d e is d e r i v a t i z e d e i t h e r m e c h a n i s m f o r n e t o x i d a t i o n of t h e s o l u t i o n species m a y y i e l d a rate t h a t is different t h a n for a n a k e d electrode. A p r i o r i , a t r u e m e d i a t e d e l e c t r o n transfer m e c h a n i s m w o u l d s e e m i n g l y y i e l d a surface

having

greater m o l e c u l a r specificity, p a r t i c u l a r l y i f t h e m e d i a t e d o x i d a t i o n o c c u r s b y w h a t w o u l d b e analogous t o a n i n n e r - s p h e r e e l e c t r o n transfer m e c h a n i s m i n v o l v i n g p r i o r c o m p l e x a t i o n of t h e s o l u t i o n r e d u c t a n t a n d t h e s u r face r e d o x reagent. F i g u r e 9 shows electroanalytical proof

for m e d i a t e d o x i d a t i o n of


s o l u t i o n ferrocene b y d e r i v a t i z e d n - t y p e S i . T h e k e y p o i n t s are as f o l l o w s . F i r s t , i l l u m i n a t i o n of the d e r i v a t i z e d n - t y p e S i i n the electrolyte s o l u t i o n i n t h e absence of f e r r o c e n e y i e l d s the u s u a l c y c l i c v o l t a m m o g r a m t h a t is i n d e p e n d e n t of w h e t h e r or n o t the e l e c t r o l y t e s o l u t i o n is s t i r r e d , since t h e e l e c t r o a c t i v e m a t e r i a l is a t t a c h e d to t h e e l e c t r o d e surface. t h e a d d i t i o n of f e r r o c e n e t o the s o l u t i o n results i n e n h a n c e d

Second,

photoanodic

c u r r e n t s u c h t h a t d i f f u s i o n l i m i t e d c u r r e n t is o b t a i n e d i n q u i e t s o l u t i o n , w h i l e a h o l e ( l i g h t i n t e n s i t y ) l i m i t e d c u r r e n t is r e a c h e d w h e n the s o l u t i o n is s t i r r e d . A n d t h i r d , w h e n the l i g h t is s w i t c h e d off at the a n o d i c l i m i t a n d t h e e l e c t r o l y t e is s t i r r e d , t h e r e is l i t t l e d e t e c t a b l e

surface-

a t t a c h e d o x i d i z e d m a t e r i a l since there is l i t t l e observable, c a t h o d i c c u r rent. T h a t is, w h e n the l i g h t is s w i t c h e d off at t h e a n o d i c l i m i t ,

hole

g e n e r a t i o n ceases a n d t h e r e d u c t a n t i n s o l u t i o n reacts w i t h t h e s u r f a c e a t t a c h e d o x i d a n t at a rate t h a t is fast c o m p a r e d w i t h t h e r e t u r n scan t i m e i n t h e d a r k . A t sufficiently fast scan rates a n d l o w c o n c e n t r a t i o n of s o l u t i o n r e d u c t a n t a c a t h o d i c r e t u r n p e a k for t h e s u r f a c e - a t t a c h e d o x i d a n t is observed.

I n p r i n c i p l e , s u c h d a t a w i l l a l l o w m e a s u r e m e n t of h e t e r o g e n e

ous e l e c t r o n transfer rates b e t w e e n t h e s o l u t i o n r e d u c t a n t a n d a t t a c h e d o x i d a n t . M e a s u r i n g t h e c o n s u m p t i o n of a t t a c h e d o x i d a n t i n t h i s m a n n e r o n a r e v e r s i b l e e l e c t r o d e is i m p o s s i b l e , since w h e n a n a t t a c h e d o x i d a n t reacts w i t h a s o l u t i o n r e d u c t a n t t h e r a t i o of s u r f a c e - a t t a c h e d o x i d i z e d t o r e d u c e d m a t e r i a l is i n s t a n t a n e o u s l y r e - e s t a b l i s h e d to a v a l u e t h a t d e p e n d s only on the electrode potential. P r o o f of m e d i a t e d o x i d a t i o n of s o l u t i o n r e d u c t a n t s other t h a n f e r r o cene has b e e n o b t a i n e d . I n p a r t i c u l a r , d e r i v a t i z e d n - t y p e S i c a n b e u s e d t o effect t h e p h o t o e l e c t r o c h e m i c a l o x i d a t i o n of F e ( C N ) electrolyte solution ( F i g u r e 10).

6

4

" i n an aqueous

U s i n g the derivatized electrode i n the

a q u e o u s e l e c t r o l y t e s o l u t i o n is i n t e r e s t i n g f o r several reasons:

efficient

d i r e c t o x i d a t i o n of a n y t h i n g i n H 0 u s i n g n a k e d n - t y p e S i is u n l i k e l y 2

o w i n g t o t h e S i O * p r o b l e m ; f e r r o c e n e c o u l d not b e u s e d as a s o l u t i o n m e d i a t o r , since i t is i n s o l u b l e i n H 0 ; a n d a n a q u e o u s e l e c t r o l y t e s o l u t i o n 2

has h i g h e r c o n d u c t i v i t y t h a n n o n a q u e o u s systems.


Derivatized


WRIGHTON E T AL.

Semiconductor

Photoelectrodes

287

Figure 9. (a) Cyclic voltammograms for n-type Si derivatized with II in stirred EtOH/O.lM [n-Bu N]ClO solutions; ( ) light switched off at + 0.25 V vs. SCE. (b) Cyclic vol tammetry for electrode in (a) in same electrolyte solution but containing 5 X 10~ M ferrocene: ( ) quiet solution with illumination for entire scan; ( ) quiet solution light switched off at + 0.25 V; ( ; stirred solution light switched off at + 0.25 V. k

4

-0.6

-0.4 -0.2 0.0 *0.2 El tetrode Potential, V vs SCE


k


288

INTERFACIAL

-0.8

-0.4 Potential,

0.0 V vs SCE

PHOTOPROCESSES

0.4

Figure 10. Cyclic voltammograms at 100 mV/sec of n-type Si derivatized with 11 in aqueous electrolytes. The electrode is illuminated with a tungsten halogen source during the anodic scans; the cathodic, return scans are in the dark, (a) Cyclic voltammograms in quiet solutions: ( ) 0.1M NaClO /H 0 showing surface waves at + 0.3 V (photooxidation) and - 0.1 V (reduction); ( ) with 9 X 10~ M K Fe(CN) added. (b) The effect of stirring on cyclic voltammogram in 9 X 10~ M K Fe(CN) electrolyte. Note the reverse scan current scale is expanded by 10 times the forward scan scale, (c) Repeat of (a); ( ) after completing (b). k

2

4

h

6

4

h


6

15.

WRIGHTON E T A L .

Derivatized

Semiconductor

Photoelectrodes

289

Use of Derivatized » - T y p e Si in a F u l l Cell Configuration; E q u i librium Current-Potential Curves.

I n sufficiently d r y E t O H e l e c t r o l y t e

solutions containing ferricenium/ferrocene w e have been able to sustain the conversion of light to electricity i n a cell like that d e p i c t e d i n Scheme I u s i n g a n a k e d n-type S i photoelectrode.

A c o m p a r i s o n of t h e e q u i

l i b r i u m c u r r e n t - p o t e n t i a l p r o p e r t i e s of a n a k e d a n d d e r i v a t i z e d e l e c t r o d e a r e g i v e n i n F i g u r e 11 f o r s u c h a c e l l . T h e p o t e n t i a l is g i v e n r e l a t i v e t o t h e S C E b u t t h e p h o t o c u r r e n t at E

r e d o x

c a n b e t a k e n as t h e u s u a l s h o r t r e d o x

reflects


circuit value; current at any potential more negative than E

-0.6 -0.4

-0.2

+ 0.2 +0.4

+0.2 +0.4 +0.6 -0.6 - 0 . 4 - Q 2 P O T E N T I A L . V vs S C E

+0.6

Figure 11. Current-voltage curves at 2 mV/sec for n - S i photoelectrode in stirred EtOH solution of 5 X 10~ M ferrocene, 2.9 X 10 M ferricenium as the PF ~ salt and 0.1 M [n-Bu N]ClO . Uniform irradiation with 632.8-nm light at the indicated power. Solution E - = + 0.32 V vs. SCE. (a) Naked electrode freshly etched with HF; (b) same electrode after derivatization with II. 2

6

3

h

h

red0


290

INTERFACIAL

PHOTOPROCESSES

c o n v e r s i o n o f l i g h t t o e l e c t r i c i t y . F r o m d a t a l i k e t h a t i n F i g u r e 11 w e c o n c l u d e t h a t t h e d e r i v a t i z e d e l e c t r o d e is c e r t a i n l y n o w o r s e , a n d p e r h a p s a l i t t l e better, t h a n t h e n a k e d e l e c t r o d e i n t e r m s o f efficiency. E a c h e l e c t r o d e suffers f r o m l o w o u t p u t voltages a t l o w l i g h t i n t e n s i t y a n d f r o m p o o r fill factors a n d q u a n t u m efficiencies a t t h e h i g h e r intensities. I m p o r t a n t l y , t h e d e r i v a t i z e d e l e c t r o d e surfaces r e m a i n i n t a c t ( c o v e r a g e a n d p e a k p o s i t i o n s ) d u r i n g t h e e x p e r i m e n t a t i o n r e p r e s e n t e d i n F i g u r e 11, as d e t e r m i n e d b y c y c l i c v o l t a m m e t r y b e f o r e a n d after r e c o r d i n g t h e d a t a i n F i g u r e 11. A s s u m i n g t h a t a l l o f t h e p h o t o c u r r e n t o c c u r s b y m e d i a t i o n , w e c l a i m t h a t e a c h a t t a c h e d r e d o x c e n t e r c a n c o n s e r v a t i v e l y pass m o r e t h a n 1 0 electrons w i t h o u t d e t e r i o r a t i o n of p r o p e r t i e s . Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch015

4

F i g u r e 12 shows t h e p h o t o c u r r e n t a g a i n s t t i m e f o r a n a k e d a n d a d e r i v a t i z e d n - t y p e S i p h o t o e l e c t r o d e i n a n aqueous s o l u t i o n of F e ( C N )

6

4

".

T h e i m p r o v e m e n t i n t h e c o n s t a n c y of t h e o u t p u t p a r a m e t e r s i n t h e d e r i v a t i z e d case is o b v i o u s . E q u i l i b r i u m c u r r e n t - p o t e n t i a l c u r v e s are s h o w n i n F i g u r e 13; s u c h curves a r e n o t o b t a i n a b l e f o r t h e n a k e d e l e c t r o d e , s i n c e t h e f o r m a t i o n of S i O * is t o o r a p i d . T h e o p t i c a l - t o - e l e c t r i c a l e n e r g y c o n v e r s i o n efficiency is s t i l l l o w i n t h e a q u e o u s e l e c t r o l y t e s o l u t i o n , b u t it s h o u l d be emphasized that t h e derivatized electrode allows sustained energy conversion whereas the n a k e d electrode undergoes decomposition

40-

3

TIME , MINUTES Figure 12. Plots of photocurrent against time for a single n - S i electrode illuminated with 632.8-nm light at ~ 6 mW. Photoelectrode held at + 0.2 V vs. SCE in stirred solutions. Supporting electrolyte is 0.1 M NaClOt in doubly distilled, deionized H 0. (A) Run 1, HF-etched naked electrode in supporting electrolyte only; (O) Run 2, naked electrode reetched with HF, in supporting electrolyte plus 4 X 1 0 " M Fe(CN) ~; (•) Run 3, electrode derivatized with II in same solution as Run 2. 2

3


4

6

15.

WRIGHTON E T AL.

Derivatized

Semiconductor

Photoelectrodes

291


+ 180 —

Figure 13. Current-voltage curves for n-Si photoelectrode derivatized with II in stirred 0.1 M Fe(CN) ' 0.01M Fe(CN) *', in doubly distilled, deionized H O. n-Si illuminated at 632.8 nm at the indicated power. Solution E = + 0.13 V vs. SCE; scan rate 5 mV/sec. 4

6

6

g

-Q4

-0.2 0 +Q2 POTENTIAL, W l

+Q4 SCE

+Q6

redow


9

292

INTERFACIAL

at a rate t h a t p r e c l u d e s s u s t a i n e d c e l l o p e r a t i o n .

PHOTOPROCESSES

T h e s e results e s t a b l i s h

the p r i n c i p l e of m a n i p u l a t i n g interface properties

for practical appli

cations. Summary n-Type S i a n d G e semiconductors

a n d A u a n d P t metal electrodes

c a n b e d e r i v a t i z e d w i t h h y d r o l y t i c a l l y u n s t a b l e ferrocenes s u c h oligomeric

amounts

o f essentially r e v e r s i b l y e l e c t r o a c t i v e

persistently attached.

I n a l l cases i t appears t h a t a t t a c h m e n t leads t o

l i t t l e c h a n g e i n t h e energetics f o r e l e c t r o n transfer. Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch015

that

material are

Derivatized

n-type

S i has b e e n u s e d to p r o v i d e d i r e c t e v i d e n c e for m e d i a t e d e l e c t r o n transfer, t h e first s u c h e v i d e n c e f o r a n y d e r i v a t i z e d e l e c t r o d e . M o s t i m p o r t a n t l y , derivatized n-type S i can b e used to illustrate the m a n i p u l a t i o n of inter f a c i a l c h a r g e transfer k i n e t i c s i n s u c h a w a y t h a t t h e d e r i v a t i z e d e l e c t r o d e is u s e f u l i n e n e r g y c o n v e r s i o n a p p l i c a t i o n s u n d e r c o n d i t i o n s w h e r e t h e n a k e d e l e c t r o d e is not. Acknowledgments W e t h a n k t h e U n i t e d States D e p a r t m e n t o f E n e r g y , Office o f B a s i c E n e r g y Sciences f o r s u p p o r t o f this research.

M S W acknowledges sup

p o r t as a D r e y f u s T e a c h e r - S c h o l a r , 1 9 7 5 - 1 9 8 0 , N S L as a J o h n a n d F a n n i e H e r t z F o u n d a t i o n F e l l o w , 1977-present, Solar E n e r g y F e l l o w , Literature

a n d M G B as a n M . I . T .

Cabot

1978-present.

Cited

1. Wrighton, M. S.; Ellis, A. B.; Wolczanski, P. T.; Morse, D. L.; Abrahamson, H . B.: Ginley, D. S. J. Am. Chem. Soc. 1976, 98, 2774. 2. Watanabe, T.; Fujishima, A.; Honda, K. Bull. Chem. Soc. Jpn. 1976, 49, 355. 3. Mavroides, J. G.; Kafalas, J. A.; Kolesar, D. F. Appl. Phys. Lett. 1976, 28, 241. 4. Heller, A.; Parkinson, B. A.; Miller, B. Conf. Bee. IEEE Photovoltaic Spec. Conf. 1978, 13, 1253. 5. Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. J. Am. Chem. Soc. 1976, 98, 1635, 6418, 6855. 6. Ellis, A. B.; Kaiser, S. W.; Bolts, T. M.; Wrighton, M. S. J. Am. Chem. Soc. 1977, 99, 2839, 2848. 7. Hodes, G.; Manassen, J.; Cahen, D. Nature (London) 1976, 261, 403. 8. Ellis, A. B.; Bolts, J. M.; Wrighton, M. S. J. Electrochem. Soc. 1977, 124, 1603. 9. Miller, B.; Heller, A. Nature (London) 1976, 262, 680. 10. Chang, K. C.; Heller, A.; Schwartz, B.; Menezes, S.; Miller, B. Science 1977, 196, 1097. 11. Heller, A.; Chang, K. C.; Miller, B. J. Electrochem. Soc. 1977, 124, 697. 12. Legg, K. D.; Ellis, A. B.; Bolts, J. M.; Wrighton, M. S. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 4116.


15.

13. 14. 15. 16. 17. 18. 19. 20.


21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.

WRIGHTON

ET

AL.

Derivatized

Semiconductor

Photoelectrodes

293

Bard, A. J.; Wrighton, M. S. J. Electrochem. Soc. 1977, 124, 1706. Gerischer, H. J. Electroanal. Chem. 1977, 82, 133. Fujishima, A.; Honda, K. Nature (London) 1972, 238, 37. Wrighton, M. S.; Ginley, D. S.; Wolczanski, P. T.; Ellis, A. B.; Morse, D. L.; Linz, A. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 1518. Nozik, A. J. Nature (London) 1975, 257, 383. Hardee, K. L.; Bard, A. J. J. Electrochem. Soc. 1975, 122, 739. Keeney, J.; Weinstein, D. H.; Haas, G. M. Nature (London) 1975, 257, 383. Fujishima, A. J.; Kohayakawa, K.; Honda, K. J. Electrochem. Soc. 1975, 122, 1487. Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 4667. Fujishima, A.; Honda, K. J. Chem. Soc. Jpn. 1971, 74, 355. Bolts, J. M.; Wrighton, M. S. J. Phys. Chem. 1976, 80, 2641. Wrighton, M. S.; Wolczanski, P. T.; Ellis, A. B. J. Solid State Chem. 1977, 22, 17. Kohl, P. A.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 7531. Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1975, 97, 7427. Laser, D.; Bard, A. J. J. Phys. Chem. 1976, 80, 459. Nakatani, K.; Matsudaira, S.; Tsubomura, H . J. Electrochem. Soc. 1978, 125, 406. Chung, Y. W.; Lo, W. J.; Somorjai, G. A. Surf. Sci. 1977, 64, 588. Ibid., 1978, 71, 199. Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Phys. Rev. Lett. 1976, 36, 1335. Watkins, B. F.; Behling, J. R.; Kariv, E.; Miller, L. L. J. Am. Chem. Soc. 1975, 97, 3549 Razena, B.; Wong, M.; Thomas, J. K. J. Am. Chem. Soc. 1978, 100, 1679. Fujihira, M.; Ohishi, N.; Osa, T. Nature (London) 1977, 268, 226. Clark, W. D. K.; Sutin, N. J. Am. Chem. Soc. 1977, 99, 4676. Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 33. Gleria, M.; Memming, R. Z. Phys. Chem. (Frankfurt am Main) 1975, 98, 303. Bolts, J. M.; Wrighton, M. S. J. Am. Chem. Soc. 1978, 100, 5257. Wrighton, M. S.; Austin, R. G.; Bocarsly, A. B.; Bolts, J. M.; Haas, O.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Am. Chem. Soc. 1978, 100, 1602. Bolts, J. M.; Bocarsly, A.; Palazzotto, M. C.; Walton, E . G.; Lewis, N. S.; Wrighton, M.S. J. Amer. Chem. Soc., 1979, 101, 1378. Pankove, J. I. "Optical Processes in Semiconductors"; Dover Publications: 1971. Wrighton, M. S.; Palazzotto, M. C.; Bocarsly, A. B.; Bolts, J. M.; Fischer, A. B.; Nadjo, L. J. Am. Chem. Soc. 1978, 100, 7264. Wrighton, M. S.; Austin, R. G.; Bocarsly, A. B.; Bolts, J. M.; Haas, O.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Electroanal. Chem. 1978, 87, 429. Gerischer, H. In "Physical Chemistry: An Advanced Treatise"; Eyring, H., Henderson, D., Jost, W., Eds.; Academic: New York, 1970; Vol. 9A, Chapter 5. Grushka, E . , Ed. "Bonded Stationary Phases in Chromatography"; Ann Arbor Science Pub.: Ann Arbor, MI, 1974. Weetall, H. H. Science 1969, 166, 615. Allum, K. G.; Hancock, R. D.; Howell, I. V.; McKenzie, S.; Pitkethly, R. C.; Robinson, P. J. J. Organomet. Chem. 1975, 87, 203. Lenhard, J. R.; Murray, R. W. J. Electroanal. Chem. 1977, 78, 195. Moses, P. R.; Murray, R. W. J. Electroanal. Chem. 1977, 77, 393. Moses, P. R.; Weir, L.; Murray, R. W.Anal.Chem. 1975, 47, 1882. Janz, G. J.; Tomkins, R. P. T. "Nonaqueous Electrolytes Handbook"; Aca demic: New York, 1973; Vol. II.


294

INTERFACIAL

PHOTOPROCESSES

52. Laviron, E . J.Electroanal.Chem. 1972, 39, 1. 53. Merz, A.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 3222. 54. Fischer, A. B.; Wrighton, M. S.; Umana, M.; Murray, R. W. J. Amer. Chem. Soc. 1979, 101, 3442. 55. Becker, E.; Tsutsui, M., Eds. "Organometallic Reactions"; Wiley: New York, 1972; Vol. IV. 56. Lenhard, J. R.; Rocklin, R.; Abrun, H.; Willman, K.; Kuo, K.; Nowak, R.; Murray, R. W. J. Am. Chem. Soc. 1978, 100, 5213. 57. Stirn, R. J.; Yeh, Y. C. M. Appl. Phys. Lett. 1975, 27, 95.


RECEIVED October 2, 1978.


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